Solid state lighting devices are well known in the art, and are rapidly replacing conventional lighting sources, such as incandescent bulbs and fluorescent lighting fixtures. As used herein, the term Light Emitting Device (LED) means a solid state lighting device, such as a light emitting diode or laser diode. The numerous advantages of LEDs over conventional lighting sources include: LEDs consume less energy to produce comparable lighting; LEDs do not generate heat on the scale of incandescent lights or even the ballasts of fluorescent fixtures; LEDs are generally fabricated with plastic or epoxy housing and lenses, and hence are more rugged; LEDs last longer than conventional light sources; LEDs do not contain toxic gases which may be released on breakage; and LEDs require no “warm-up” and may be cycled at high frequencies.
Due to these advantages, light fixtures, also known as luminaires, became commercially available soon after the development of practical white light LEDs. As depicted in FIG. 1, these luminaires 10 typically assemble a homogenous plurality of white LEDs in series into a single string 12, driven by a power supply 14 that provides a substantially constant drive current. These power supplies 14 typically include a control circuit that varies the output voltage as necessary to attempt to supply a constant current to the fixed load of the LED string 12. Many such power supplies 14 include a “dimming” or brightness control feature, whereby the constant output current can be adjusted, either in discrete steps, or continuously over a range, to drive the LED string 12 at different levels of illumination (also referred to herein as luminous flux). As used herein, the term “constant current power supply” refers to a power supply circuit that attempts to output a substantially constant current to a load, at any particular selected drive (brightness) level. The constant current power supply 14 typically includes a feedback control loop which monitors the current consumed by a load, and varies the output voltage to maintain the current at a substantially constant value. Of course, in the real world, fluctuations in the output current are always present, due to supply power variations; component heating, aging, or tolerances; variations in individual LEDs; changes in temperature or humidity; and the like.
Light sources—both individual LEDs and luminaires constructed with them—may be characterized by various luminous characteristics, such as Color Rendering Index (CRI), chromaticity, Correlated Color Temperature (CCT), and luminous flux.
The CRI of a light source is a measure of the ability of the light generated by the source to accurately illuminate a broad range of colors. Light sources are “indexed” to the color rendering properties of a black-body radiator. A CRI of 100 indicates that the perceived color of a set of test colors being illuminated by the light source are the same as if the test colors were irradiated by the black-body radiator. Daylight has a CRI of 100; incandescent light bulbs have a CRI of around 95; fluorescent lamps have a CRI of around 70-85; and monochromatic light sources have a CRI close to zero.
The chromaticity, or color, of a light source may be expressed as a “color point” in a coordinate system of a color space, such as a tristimulus value (X, Y, Z) or the color coordinates (CCx, CCy) on a chromaticity diagram. Alternatively, because a black-body radiator emits a range of colors depending on its temperature, color information may also be expressed as a CCT, which is the temperature (on the Kelvin scale) at which the heated black-body radiator matches the color of the light source. The CCT of white light sources ranges from around 2700 K to 6500 K. Light at the lower end, around 2700 K, has a yellowish color (referred to as “warm” white light), and light at the upper end, around 6500 K, has a blueish color (referred to as “cool” white light).
The luminous flux of a light source refers to the intensity of its illumination, measured in lumens, where one lumen is the amount of light emitted per second in a unit solid angle of one steradian from a uniform source of one candela. The luminous flux of most LEDs varies with the current with which it is driven; however, LEDs may be manufactured to different sizes and hence may have different luminous flux capabilities.
As depicted in FIG. 1, the drive circuits 10 of early LED luminaires typically included a single string 12 of homogeneous white light LEDs, all outputting the same luminous characteristics, and all driven together in series. For example, all LEDs in such a luminaire may be white Light Emitting Diodes. A white LED is a blue LED that includes a phosphor which absorbs some of the high-energy blue light and emits lower-energy yellow, green, and red light. The phosphor-converted light mixes with an unchanged portion of the blue light, producing perceptually white light. White LEDs typically have either a poor CRI and high efficacy, or a high CRI and poor efficacy, but rarely can achieve both high CRI and efficacy. The single-string luminaire has a set chromaticity/CCT, depending on the particular LEDs used, which are typically on the cool end of the spectrum.
In contrast, FIG. 2 depicts a circuit configuration 20 typical of more modern LED luminaires. These luminaires mix, or blend, the light from a plurality of heterogeneous strings 24, 28, of LEDs, and independently control the intensity of each string. In particular, a first string 24 comprises Blue-Shifted-Yellow (BSY) LEDs. Similarly to the white LEDs described above, BSY LEDs emit blue light, and phosphors shift some of the light to the yellow range of the spectrum; the combination of blue and yellow produces a cool white light. The first string 24 of BSY LEDs is driven by a first constant current regulator 26, such as a buck regulator, which controls the luminous flux of light generated by the first string 24. A second string 28 of, e.g., red or red-orange (RDO) LEDs, connected in parallel with the first string 24, is driven by a second constant current regulator 30, which independently controls the luminous flux of light generated by the second string 28. Both current regulators 26, 30 are controlled by a controller 32, such as a microprocessor or microcontroller, which controls the overall, mixed, light output by independently controlling the intensity of the two (or more) LED strings 24, 28. The ratio of intensities may be predetermined, user programmed, or dynamically changed in response to various inputs or sensed conditions. A constant voltage power supply 22 provides power for all of the luminaire components.
In the circuit 20, luminous characteristics such as CRI and CCT may be tailored to specific applications, or preset in ranges that have been shown to maximize user comfort and acceptance. For example, the TRUEWHITE® technology lighting available from Cree, Inc. of Durham, N.C. combines white light LEDs, such as BSY, with RDO and unsaturated yellow LEDs, to create a warm white light with a CRI of over 90. Additionally, the relative drive strengths of different LED strings may be dynamically adjusted to produce a variety of effects. For example, Cree's sunset dimming technology alters the CCT of a light source as a function of its dimming level, mimicking incandescent lighting.
A variety of highly integrated, low-cost, constant current power supplies 14 are commercially available, in form factors designed for use in solid state luminaires. Furthermore, many of them include integrated dimming capability, such as from user input, remote control, sensing ambient lighting conditions, and the like. In many cases, it would be advantageous to utilize these constant current power supplies 14 with the advanced heterogeneous, multi-string configurations 20, to achieve the above-stated advantages of controlling luminous characteristics, such as CRI, CCT, and the like. However, since modern LED lighting circuits 20 are designed to operate from a constant voltage power source 22, it is not possible to use these constant current power supplies 14 directly.
The Background section of this document is provided to place embodiments of the present invention in technological and operational context, to assist those of skill in the art in understanding their scope and utility. Unless explicitly identified as such, no statement herein is admitted to be prior art merely by its inclusion in the Background section.